The slippery slope here is not that people will stop thinking, observing, or even protesting. Rather, it's that with the laws and infrastructure in place, it becomes very easy to block more pages. So rather than a uncontrolled "slippery slope", maybe it should just be called a "first step" or sure tell-tale sign.
That idea is not based on some illogical extrapolation into the future, but on a number of examples from the past: Many European countries have already followed this pattern, some of which you'd think be among the most liberal: Holland, Denmark, France, Germany, UK. Take UK as a prime example: First they put in place the infrastructure because of child porn. Then it's used against "hate speech" or "terrorism". Next violent porn, BDSM. After that, file sharing sites, The Pirate Bay. Next, political party sites like The Pirate Party, and the Chaos Computer Club. The latter two have already been "mistakenly" blocked in multiple countries.
Now, many people believe the state should not be in the business of policing the Internet. So in the UK, they've made the brilliant move of making it "voluntarily". All the major ISPs now have personal filters controlled by their customers. Of course, it'd be a bit naive to think that those settings could not be used against you: If you ever find yourself in a sexual abuse case, Child Protection Services case, background check / government security clearance, you'd better have those settings in the right position.